The possible secret presence of alien reconnaissance probes in the solar system is discussed in the scientific literature (e.g. Frietas, 1983; Graham, 1990; Arkhipov, 1991; Holmes, 1991). Our satellite could be a convenient base for monitoring of the Earth by some extraterrestrial civilization. There are important advantages of a probe landed on the moon relative to an orbiting probe or one landed on the Earth (Arkhipov, 1993):

The maximum lifetime of probes is at least doubled because the moon shields the device from meteoroids for 2 steradians.
The electronic devices work more stable and for longer times because the moon shields half of the cosmic ray flux.
The unfavorable atmospheric, geological and biological factors of the Earth are lacked on the moon.
The problem of stabilization is simplified radically for a landed probe vs. an orbiting one.
The camouflage of the mission.

Of course, the "presumption of naturality", the main method of current SETI programs, is ineffective for a search of camouflaged activity (Rubtsov, 1991). But the well-known intelligence method of provocation for responsive reaction seems quite pertinent.

INVASION EFFECT

Fortunately, the incursions of our probes and spacecraft on ihe lunar surface could be used as such provocations. It is important that some lunar transient phenomena (LTP) seem to be correlated with our activity on the moon. Let me first note the most convincing cases:

Already the first impact of "Luna 2" and its booster on September 13, 1959 was accompanied by simultaneous LTP (flashes, clouds) from at least four different sites (Fielder, 1960). The reports about that LTP were confirmed.
Only one LTP (Cameron,1978, No. 393) was observedin the Mare Tranquilitatis (except the Mare Serenitatis Border) before the first probe impact of Ranger 6 on February 2, 1964, about 50 km to the south of the crater Ross. The probable dust cloud appeared in Ross several hours after that event (Cameron, 1978, No. 799). Then the 100 km x 100 km zone to the east from Ross (Fig. 1) became one of the most LTP-active regions on the moon up to September 25, 1969. The second LTP-zone appeared between craters Sabine and Maskelyne just after Surveyor 5 landed there on September 11, 1967. After 1969 LTP activity in the Mare Tranquilitatis area vanished (Fig. 2a). The increasing number of LTP reports in the 1960's cannot explain this phenomena. About 7.7% of current LTP belong to this Mare in 1964-69, but 0.1% only at other times. This local "wave" of LTP reports is statistically significant Indeed, n=31 LTP in the Mare (in the borders from Fig.1) were reported from February 2, 1964 to September 25, 1969 (Cameron,1978). N=404 events were noted on the whole lunar disk at that time interval. But for the total time 32 reports from 1464 belong to the Sea. Therefore, the average probability of LTP in the Mare is p=32/1464. Hence, the probability of higher accidental fluctuation is:
The reports about LTP in the crater Gassendi were most frequent during 1966-67 when Surveyor 1 landed at the shortest distance (500 km ) from that crater (Fig. 2b). According to the equation, the probability of such accidental fluctuation is W₂= 5.3x10^-8 (with n=11; N=50; p=38/1464, Cameron, 1978).

It is scarcely likely that the observers were interested especially in the landing sites. So, the LTP-zone near Ross D existed for 5 years while 16 probes landed in other sites. Nevertheless, LTP activity in the zone was not noted during the Apollo 17 landing on December 11, 1972 at 370 km from Ross. Although crater Ross D is recommended for LTP patrol (Sytinskaya,1981), the reports about transient lunar events here are very rare now. In addition, the landings on Mare Foecunditatis, Mare Imbrium, Mare Serentitatis, and Oceanus Procellarum, are not accompanied by significant increase in LTP-activity there. Our exploration of the Mare Imbrium in 1959-1971 even coincided to the temporary damping of LTP-activity of Mt. Pico in that region during 1956-71 (Cameron, 1978).

Therefore, the "Invasion Effect" in some cases seems quite real.

POSSIBLE INTERPRETATION

In case of secret monitoring of the Earth, the extraterrestrial beings must shun contact with us. The ideal method of camouflage on the moon is an artificial explosion for a destruction of tracks and a dust screen. This action appears as a meteorite impact forming a common crater. An explosion dust cloud could be observable on the Earth if its mass is >1000kg. only (Geak and Mills, 1977). But usually the invasion effect appears just as cloud-like phenomena or reddish spots. These LTP are interpreted as dust or dust-gas clouds (Geak and Mills 1977). So, some of the like LTP could be artificial. Hence the correlation of LTP with time and sites of some landings of probes on the moon could be a reaction of intelligence on our incursion to some important lunar region. To the point, the crater Gassendi has already been picked out as a promising site for exo-archaeological exploration (Arkhipov, 1993).

CONCLUSIONS

The incursions of Earth's vehicles in certain lunar regions (Mare Tranquilitatis, Gassendi, etc.) stimulate a statistically significant, real, temporary increasing probability of lunar transient phenomena there. The observational selection can not be an adequate explanation of this effect. But, the induced LTP could be elementarily interpreted in the terms of a hidden alien presence on the moon. However, there is a psychological prejudice against this approach. Is this correct? What is more reasonable: to consider the shallow Earthquakes in Nevada as natural phenomena or nuclear cxplosions? Obviously, the second hypothesis is more prohable. But, how do we estimate the possibility for the lunar case a priori? That is why the "artificial" interpretation of the "Invasion Effect" is worth discussion. The methods of intelligence serviceare more adequate for a search for reticent extraterrestrial intelligence than those of astronomy, selenology and geology directed to study of natural processes or objects. It is better to be on alert than carelessly ignore an alien presence.

REFERENCES

Arkhipov, A.V., 1991. "Radio Search for Alien Space Probes" in J. Heidmann, M.J, Klein, Eds., Bioastronomy, The Search for Extraterrestrial Life -The Exploration Broadens. Heidelberg: Springer- Verlag, pp. 244-246.

Arkhipov, A.V., 1993 "The Moon as an Attractor of Alien Artifacts", Selenology, 12, No.1, pp. 6-8.

Cameron, W.S. 1977, "Lunar Transient Phenomena (LTP): Manifestation, Site Distribution, Correlations and Possible Causes", Physics of the Earth and Planetary Interiors, 14, No. 2, pp. 194-216

Cameron, W.S. 1978 Lunar Transient Phenomena Catalog. National Space Science Data Center, Greenbelt: NASA.

Fielder, G., 1960. "Observations Related to the I mpact of Lunik II, Nature,185 , p. 11.

Frietas, R.A.,1983. If they are Here, Where are They? Observational and Search Considerations", Icarus, 55, pp. 337-343.

Geak, J.E., and Mills, A.A. Possible Physical ProcessesCausing Transient Lunar Events", Physics of theEarth and Planetary Interiors,14 (3),299-320

Graham, F.G. 1990 "Anomalous Pixels in Spacecraft Multispectral Images and the Possibility of Memento Artifacts on the Moon", Selenology, 9, No. 4, pp. 18-23.

Holmes, D.L 1991. "Archaeology in Space: Encountering Alien Trash and Other Remains", in: J. Heidmann, M.J. Klein, eds., Bioastronomy. The Search for Exrraterrestrial Life - The Exploration Broadens . Heidelberg: Springer-Verlag, pp. 327-332.

Rubtsov, V.V. 1991. Criteria of Artificiality in SETI" in:J. Heidmann,M.J. Klein, eds., Bioastronomy: The Search for Extraterrestrial Life - The Exploration Broadens. Heidelberg: Springer-Verlag, pp. 306-310.

Sytinskaya, N.N. 1981. "The Instructions for Lunar Observations", in: V.K. Abalakin, ed., The Astronomical Calendar, Constant Part. Moscow:Nauka, pp. 323-332 (in Russian).

Vitkevich, V.V., Kuzmin,A.D., Sorochenko,R.I, and Udal'tsov, V.A., 1960. Radioastronomic Observations of the Second Soviet Space Rocket", Reports of the Academy of Sciences of the USSR, 132, 1, pp. 85-88 (in Russian).